dm writecache: skip writecache_wait when using pmem mode

The array bio_in_progress is only used with ssd mode. So skip
writecache_wait_for_ios in writecache_discard when pmem mode.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>